1. Field of the Invention
The present invention relates to a head driving device for moving a head for recording/reproducing data in the radial direction of a disk and placing this in a target position and a method for driving the same in a disk recording/reproducing device, for instance a hard disk drive.
2. Description of the Related Art
Conventionally, especially a compact hard disk drive (HDD) has been utilized as a built-in type external storage device for the personal computer of a laptop type or a notebook type. In recent years, with attainment of high performance for the personal computer, there has been an increase in demands for compact and large storage capacity HDDs.
In order to increase the storage capacity of the HDD, it is necessary to improve the track density and linear recording density of a disk as a recording medium so as to attain the high recording density of the disk. With the attainment of such a high recording density, a technology must be provided, whereby a head may be highly accurately positioned in a target position (track) to be accessed on the disk.
In the HDD, head positioning control is executed by means of a servo system for typically performing speed control and position control. The servo system positions the head in the target position by drive-controlling a head actuator for supporting the head and moving this in the radial direction of the disk.
Accordingly, in order to effectuate highly accurate positioning control for the head, various ideas have been tried to improve the mechanism of the head actuator in addition to a control technique.
As shown in FIG. 1, the conventional compact HDD incorporates a rotary and oscillatory type head actuator 1 constituting a disk drive, a disk 101, a spindle motor 102, a voice coil motor (VCM) 2 and a circuit substrate 103 in a case 100 made of, for instance an aluminum alloy.
The disk 101 rotates at a high speed, being driven by the spindle motor 102. The circuit substrate 103 is provided with various circuit parts, for instance a head amplifier for amplifying a read signal from a head 3.
The head actuator 1 is constructed by a suspension 4 for supporting the head 3, a support arm (head arm) 5 for supporting the suspension 4 and transmitting a rotary driving force and an actuator body 6 supporting the support arm 5 and driven to rotate around a rotary shaft 6a by means of a driving force provided by the VCM 2.
A plurality of disks 101 are normally provided in the HDD and heads 3 are disposed in both surfaces of each disk 101. Accordingly, a suspension 4 is provided for each head 3. Support arms 5 are provided according to the number of heads 3 so as to be smaller in number than the latter.
The head 3 is mounted in a slider. By means of the floating movement of this slider, a reading/writing operation is performed with a constant tiny gap from the surface of the disk 101.
The VCM 2 is constructed by a driving coil provided in a coil frame having an almost V-shaped structure, not shown, a permanent magnet and opposing yokes. The permanent magnet and the opposing yokes constituting a magnetic circuit are disposed in upper and lower directions so as to sandwich the driving coil held on the coil frame of the support arm 5. The permanent magnet is supported by a holding yoke.
The VCM 2 conducts electricity to the driving coil placed within a magnetic field produced between the driving coil and the opposing yokes and an electro-magnetic force produced in the driving coil causes the support arm 5 of the actuator to rotate and oscillate around the rotary shaft 6a. A rotary and oscillatory ball bearing is provided in the actuator body 6.
The servo system controls the driving current of the VCM 2 so as to execute drive-controlling of the head actuator based on servo data recorded beforehand in the servo area of the disk 101.
As a cause of failure when by highly accurately driving the head actuator, the head 3 is to be positioned in the target position with high accuracy, mechanical vibrations occurring in components, for instance the support arm 5, constituting the head actuator may be cited.
When the servo system supplies a current to the driving coil of the VCM 2 and drives the head actuator in the radial direction of the disk, a transmission characteristic on a frequency with the position error signal (burst data) of the servo data (displacing response to a force and called compliance) will be like that shown in FIGS. 2A and 2B.
The position error signal is servo data used for the above-noted position control operation and detecting the position error of the head 3 from the center of the track.
In this case, a gain peak due to resonance is reached largely because of the rolling friction characteristic of the ball bearing provided in the actuator body 6 in the vicinity of 100 Hz and the main resonance mode of the actuator structure itself in the vicinity of 4 kHz. FIGS. 3A to 3C are views respectively showing a modified main resonance mode in the vicinity of the frequency 4 kHz of the transmission characteristic shown in FIGS. 2A and 2B. This is an analytic model obtained by using a computer.
This analytic model enables us to assume that the main resonance mode of the actuator structure itself will be produced by a mode for moving the driving coil of the VCM 2 and the coil frame holding the same in the rotational direction of the disk 101.
Vibrations generated due to the actuator structure in such a high frequency of 1 kHz or higher may adversely affect the servo system for performing position control for the head 3, causing an erroneous operation, for instance off-track.
In other words, vibrations due to the actuator structure may cause reduction in accuracy for positioning the head 3 and the recording density of, especially a track direction (track density). Thus, in order to reduce adverse influence on the servo system as much as possible, in the head driving device, as many mechanical vibrations as possible due to unnecessary structures must be eliminated while the resonance vibrations of the support arm 5 and the like must be increased.
In order to increase the track density, it is desired to make track pitches as small as possible. It is necessary to increase the track follow-up ability of the head 3 (positioning characteristic with respect to the track center) so as to increase reliability of data recording/reproducing (reduction in the occurrence possibility of reading errors). Usually, track errors (assuming that an off-track amount is about three times as standard deviation) must be set to a level of 0.07 times with respect to the track pitch.
Furthermore, it is necessary to reduce vibrations from the outside of the HDD and off-track against an external force from the spindle motor 102 as much as possible. For this purpose, a gain cross frequency (frequency when the gain crosses 0 dB) in the open loop characteristic of the servo system must be increased as much as possible.
As a method for increasing such a gain cross frequency in the open loop characteristic of the servo system, reference may be made to, for instance Japanese Patent Application KOKAI Publication No. 51-36924 or "Track Following Control of 2-Stage Access Servo System for Magnetic Disk Drive", Electronics Information and Communication Society Thesis Magazine (Vol. J75, No. 11, p 653 to 662).
In the Publication and the Reference, there was presented a method of providing, in addition to a main actuator mechanism (driven by the VCM 2) for integrally (simultaneously) moving a plurality of heads by long strokes, an sub-actuator (constituted of a piezoelectric element) for fine-moving each head independently.
This method is also described in U.S. Pat. No. 3,924,268. According to the method of this Publication, the head is caused to follow-up narrow track pitches by a dual servo system such that the main actuator in the heavy mass section can move and the light sub-actuator can move in a high frequency band.
A method having a subactuator mounted for each support arm for supporting the head is described in Japanese Patent Application KOKAI Publications Nos. 51-36924, 3-69072, 3-102684 and 3-183070. According to this method, each head can individually move by the subactuator.
In the Japanese Patent Application KOKAI Publications Nos. 3-69072, 3-102684 and 3-183070, a reference is made to the case of a servo face servo system, wherein the subactuator is used as means for correcting heat off-track.
Heat off-track occurs because of deformation of each of a plurality of support arms in the radial direction of the disk due to a temperature increase inside and outside the device. For this reason, means for correcting the data head only by DC components based on the positional information of the servo face and the slight sector information for each data surface is utilized. This makes it necessary to dispose the subactuator for each support arm and to control each subactuator.
FIG. 4 is a view showing the concept of representing an actuator mechanism comprising the main actuator and the sub-actuator as a spring and material point system simple model (vibration model) in each actuator section.
This model assumes that the driving coil 111 of the VCM 2 has a mass m1, an electromagnetic force F1 acts on this material point and the coil is coupled with the rotary center 113 of the actuator and caused to rotate and move by means of the spring K1 of the coil frame. In the rotary center 113, a spring Ka due to a rolling frictional force by the ball bearing and the moment J of polar inertia of the actuator are provided. A piezoelectric element model 114 equivalent to the sub-actuator is provided in the tip of an arm length L2 from the rotary center 113 of the actuator.
A translation force F2 is applied to both ends of this section, with the piezoelectric element 114 acting in a tensile (or compressing) direction. A model 116 for the support arm section and the suspension section is provided in a side opposite the rotary center 113.
This model 116 is represented by a mass m3 and a spring k3, assuming that the support arm and the suspension constitute a single unit.
In the drawing, c1 and c2 represent viscosity attenuation, x1 the displacement of the driving coil 111 in the translation direction, x2 the displacement of the piezoelectric element model 114, x3 the displacement of the head point and .theta. the rotary angle displacement of the actuator around the rotary center 113.
A rod must be rigid with no mass. x4 represents "x4=-L1.theta.", x5 "x5=L2.theta." and x6 "x6=x2+L3.theta.".
In such a vibration model, the following relational expressions (Expressions 1 to 4) hold true. ##EQU1## where, KA=ka+k1L1.sup.2 +k2L2.sup.2 +k3L3.sup.2 .omega..sub.1.sup.2 =k1/m1, .omega..sub.2.sup.2 =k2/m2, .omega..sub.3.sup.2 =k3/m3, .omega..sub.a.sup.2 =ka/J .omega..sub.a1.sup.2 =k1L1.sup.2 /J, .omega..sub.a2.sup.2 =k2L2.sup.2 /J, .omega..sub.a3.sup.2 =k3L3.sup.2 /J
If a response {X} is to be obtained by Laplace-transforming the above expression, the result will be as follows (S in the expression represents a Laplace operator): EQU {X}=[[M]S.sup.2 +[C]S+[K]].sup.-1 {F} (2)
If 0-order approximation is to be realized by assuming that viscosity attenuation items are all 0 and if attention is to be paid to the response displacement of the head x3 when the electromagnetic force F1 acts on the driving coil 111 of the main actuator and when the electromagnetic force F2 acts on the piezoelectric element 114 of the sub-actuator, the expression is as follows. In the expression, det represents a characteristic value. Head responses x3 (S) by the main actuator are as follows respectively. ##EQU2##
Herein, parameters for the model are set as follows from the actual actuator mechanism:
m1=2.5.times.10.sup.-3 (kg), .omega..sub.1 =25133 (rad/sec)
m2=0.01.times.10.sup.-3 (kg), .omega..sub.2 =125664 (rad/sec)
m3=0.5.times.10.sup.-3 (kg), .omega..sub.3 =753982 (rad/sec)
J=0.5.times.10.sup.-6 (kgm.sup.2), .omega..sub.a =251 (rad/sec)
L1=15.times.10.sup.-3 (m), L2=5.times.10.sup.-3 (m),
L3=40.times.10.sup.-3 (m)
By using these parameters, according to the expression (3), the transmission characteristics of head response displacement by the main actuator will be gain and phase characteristics like those shown in FIGS. 5A and 5B. Also, according to the expression (4), the transmission characteristics of head response displacement by the sub-actuator will be gain and phase characteristics like those shown in FIGS. 5C and 5D.
In FIGS. 5A to 5D, since there is no damping due to viscosity attenuation, gains by resonance will be high. However, there will be almost no change in frequencies having peaks in the gains, depending on the existence of damping.
The transmission characteristics of FIGS. 5A and 5B have tendencies similar to those shown in FIGS. 2A and 2B and spring resonance occurs due to the rolling friction of the ball bearing in the vicinity of a frequency below 100 Hz. When this resonance frequency is exceeded, the gains will be reduced at -40 dB/dec. At about 3 kHz, the gains will reach their peaks in the mode of the mechanical main resonance of the whole actuator structure. This resonance mode can be assumed to occur because of m1 and K1 of the driving coil 111.
FIGS. 5C and 5D respectively illustrate compliance by the sub-actuator. Gains will be almost flat until the mechanical main resonance mode of the whole actuator structure at 3 kHz. It can be understood, however, that the mechanical main resonance mode of the whole actuator structure is excited by driving the sub-actuator. This may be assumed to occur because of connection between the main actuator and the sub-actuator by means of a serial spring.
It is clear from the above description that when the sub-actuator is to be utilized, it is necessary to increase the mechanical main resonance mode of the entire actuator structure. In other words, in order to keep the gains flat up to the region of the high frequency band by the sub-actuator, it is necessary to provide a component as a notch filter in the servo system with respect to this resonance vibration number.
In recent years, digital servo system using a high-speed microprocessor has been developed for the compact HDD. This has made it easier to mount the notch filter. However, frequencies in the mechanical main resonance mode of the entire actuator structure vary among the devices and may change with the passage of time (secular change). This makes it extremely difficult to set the notch frequency of the notch filter (frequency not be passed through the filter). As a result, since positioning error amounts vary among the devices and increase during use, there is a possibility that it will be impossible to perform data recording/reproducing.
As described above, in the conventional head driving device equipped with the main actuator and the sub-actuator, the sub-actuator (piezoelectric element) is provided for each support arm. In such a structure, the number of piezoelectric elements provided to be driving means as the sub-actuators must be equivalent to that of support arms. Thus, in case where characteristics vary among the piezoelectric elements, head fine-movements and head (slider) floating characteristics may probably occur.
Cables and driving circuits for driving the sub-actuators must also be provided in the same number as that of support arms. This may complicate controlling of the individual sub-actuators by a control method.
Furthermore, since the sub-actuator is driven for each support arm, it is impossible to induce the main resonance mode of the entire actuator structure so as to increase a servo band in the head positioning system.